Pallets for transporting and storing goods have been constructed of various materials including wood, steel and plastic. Plastic pallets in particular have gained increased acceptance due to factors such as their high strength-to weight ratio, resistance to corrosion, and durability. While numerous techniques for producing plastic pallets are known, it has become increasingly popular to thermoform plastic pallets, especially with a technique known as twin sheet thermoforming as discussed in U.S. Pat. No. 3,583,036, U.S. Pat. No. 3,787,158 and U.S. Pat. No. 3,925,140, all to Brown.
Twin sheet plastic pallets are designed to take maximum advantage of the materials used. Efforts have been made to maximize the load carrying capacity of the pallet for the given amount of material used in forming the pallet. U.S. Pat. No. 3,187,691 to Leitzel, for example, shows a pallet having linearly extending channels or ribs which are formed into the pallet to increase stiffness. These ribs or channels can have an undesirable effect of allowing bending or hinge moments to occur along the length of the rib, i.e. the material may flex about an axis determined by the length of the rib or channel.
Attempts to counteract this effect include U.S. Pat. No. 3,610,173 to McIlwraith, which shows a plastic pallet having thermoplastic top and bottom decks which are releasably assembled together in spaced apart relation by a plurality of spaced column members. Each of the decks has parallel linear channels in one surface, and a series of parallel channels running at an angle to the first series of channels in the opposite surface.
Structure such as the bottom deck shown in the McIlwraith '173 patent is advantageous to distribute over a wider area the focused pressure transfered by the legs of a loaded pallet, and thus minimize or eliminate unwanted damage when the pallets are stacked. U.S. Pat. No. 5,197,396 to Breezer et al., for example, also shows a double deck plastic pallet including a twin sheet thermoformed lower deck. The lower deck supports the pallet and its contents when the pallet is in storage either on a supporting surface or stacked on another loaded pallet.